Tests: Add a test verifying that ptrace self-attaching fails

Tests/Kernel/CMakeLists.txt

@@ -50,6 +50,7 @@ set(LIBTEST_BASED_SOURCES
     TestPosixFallocate.cpp
     TestPosixSpawn.cpp
     TestPrivateInodeVMObject.cpp
+    TestPtrace.cpp
     TestKernelAlarm.cpp
     TestKernelFilePermissions.cpp
     TestKernelPledge.cpp

Tests/Kernel/TestPtrace.cpp

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2026, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Atomic.h>
+#include <LibTest/TestCase.h>
+#include <errno.h>
+#include <pthread.h>
+#include <signal.h>
+#include <sys/ptrace.h>
+#include <unistd.h>
+
+static Atomic<bool> s_exit_thread = false;
+
+static void* thread_entry(void*)
+{
+    while (!s_exit_thread)
+        continue;
+
+    return nullptr;
+}
+
+TEST_CASE(ptrace_self_attach_fail)
+{
+    pthread_t thread = 0;
+    VERIFY(pthread_create(&thread, nullptr, thread_entry, nullptr) == 0);
+    VERIFY(thread > 0);
+
+    int ptrace_return = ptrace(PT_ATTACH, thread, 0, 0);
+    int error = errno;
+    EXPECT_EQ(ptrace_return, -1);
+    EXPECT_EQ(error, EPERM);
+
+    s_exit_thread = true;
+    VERIFY(pthread_join(thread, nullptr) == 0);
+}